It doesn't matter if there were 2,000 or 20,000. Even if the local Vulcan population was completely pacificist the siege would have been broken in less than six months.
Orbiting starships using a combination of transporters and large area phaser stuns would have eaten away at the invasion force.
Was this force all grunts? How many were officers and technicians? The loss of key personal would have weakened the overall effort.
The whole idea was stupid. I imagine Sella was rightfully put to death for its failure.